1. Field of the Invention
The present invention relates to an organic EL element drive circuit and an organic EL display device using the same organic EL element drive circuit. In particular, the present invention relates to an organic EL element drive circuit capable of reducing variation of drive current in a driver IC for current-driving an organic EL panel for use in a portable telephone set, etc., and reducing luminous variation on a screen of an organic EL display device due to difference in characteristics between driver ICs and, particularly, suitable for a high luminous color display and an organic EL display device using the same organic EL element drive circuit.
2. Description of the Related Art
Since an organic EL display device can perform a high luminance display due to spontaneous light emission, the organic EL display device is suitable for use in a display device whose display screen size is small and is expected as the next generation display device to be mounted on such as a portable telephone set, a DVD player or a PDA (personal digital assistance), etc. A known problem of the organic EL display device is that variation of luminance becomes considerable when a voltage drive is applied to the organic EL display device as in a liquid crystal display device and the drive control becomes difficult due to the difference in sensitivity between R (red), G (green) and B (blue).
In view of this problem, an organic EL display device using a current driver is proposed recently. For example, in JP H10-112391A, a technique for solving the problem of luminance variation by employing the current drive is disclosed.
In a recent organic EL display panel of a passive type organic EL display device for use in a portable telephone set, the number of terminal pins of column lines (anode side drive lines of organic EL elements) is 396 (132×3) and the number of terminal pins of row lines is 162. These numbers of the terminal pins are still increasing.
With such increase of the number of terminal pins, the number of column IC drivers is three currently and the number of terminal pins of each driver for one of R, G and B display colors in the case of QVGA full color display is 120, so that the total number of the terminal pins of the three drivers becomes 360. Therefore, there is a problem that luminance variation occurs on a screen of an organic EL display device due to difference in characteristics between the column IC drivers, particularly, due to variation of drive circuits thereof.
For example, JP2001-42827A discloses a technique for solving the above problem.
FIG. 3 is a circuit diagram disclosed in JP2001-42827A. In FIG. 3, an initial stage column IC driver (anode line drive circuit as a master chip) 21 includes a reference current control circuit RC, a control current output circuit CO, a switch block SB having switches S1 to Sm and circuits composed of transistors Q1 to Qm and bias resistors R1 to Rm and provided correspondingly to the terminal pins as in current drive sources. A next stage column driver IC (a second anode line drive circuit of a slave chip) 22 includes a reference current control circuit CC, a switch block SB having switches S1 to Sm and circuits composed of transistors Q1 to Qm and bias resistors R1 to Rm and provided correspondingly to the terminal pins as m current drive sources. The m current drive sources are constructed with transistors Q1 to Qm and resistors R1 to Rm, respectively. Output currents i of the transistors Q1 to Qm of the drivers are supplied to the pins through the switches S1 to Sm and output terminals X1 to Xm, respectively.
The reference current control circuit RC is constructed with an operational amplifier OP supplied with a reference voltage VREF, a transistor Qa, which is driven by an output of the operational amplifier OP supplied to a base thereof, a resistor Rp provided between an emitter of the transistor Qa and ground and a transistor Qb having collector connected to a collector of the transistor Qa on an upstream side of the transistor Qa. A voltage generated by the resistor Rp is fed back to an input of the operational amplifier OP, so that the reference current control circuit constitutes a constant current source. An emitter of the transistor Qb is connected to a power source line VBE (corresponding to a power source line VDD of the display device) through a resistor Rr.
A current mirror circuit is constructed with the transistor Qb as an input side transistor and the transistors Q1 to Qm and a transistor Qo of the control current output circuit CO as output side transistors. The transistor Qb is driven by a reference current IREF generated by the reference current control circuit RC.
The drive current control circuit CC of the column driver IC 22 corresponds to the reference current control circuit RC. The drive current control circuit CC is constructed with a current mirror circuit including transistors Qc and Qd and a transistor Qe driven by the output side transistor Qd of the current mirror circuit. The input side transistor Qc of the column driver IC 22 is supplied with an output current Iout=ic of the control current output circuit CO of the column driver IC 21 to drive the transistor Qe of the column driver IC 22. The transistor Qe of the column driver IC 22 is an input side transistor of a current mirror circuit constituted with the transistors Q1 to Qm. Resistance values of the resistors Ro and Rr are equal and a resistance value of the resistor Rs is equal to a value of the parallel resistors R1 to Rm. The switches S1 to Sm of the switch block SB of the column driver IC 21 are ON/OFF controlled by control signals GA1 to GAm and the switches S1 to Sm of the switch block SB of the column driver IC 22 are ON/OFF controlled by control signals GB1 to GBm.
As another organic EL drive circuit having a construction similar to that shown in FIG. 3, a pair of current mirror circuits having an input side transistor and output side transistors are provided in a position corresponding to the switch block SB. In the current drive circuit, input side transistors are provided correspondingly to terminal pins and. The switching operation of the current drive circuit is ON/OFF controlled by the control signals GA1 to GAm.
Further, JPH9-232074A and JP2001-143867A disclose techniques, in each of which a D/A converter circuit is provided in an upstream side of a current mirror output circuit such as shown in FIG. 3 and generates drive currents for the respective terminal pins by D/A converting the display data for column side terminal pins of an organic EL display device.
A problem of the current drive circuit, in which the current mirror circuit for driving a plurality of output side transistors in parallel is used in the drive stage or the output stage will be described with reference to the column driver ICs 21 and 22 shown in FIG. 3.
In the organic EL drive circuit shown in FIG. 3, the output current Iout=ic of the transistor Qo of the column driver IC circuit 21 is supplied to the transistor Qe of the column driver IC 22 through the current mirror transistors Qc and Qd. Therefore, the output current i of the current mirror circuit is equal to the reference current IREF theoretically. However, even if the reference currents of the chips are made equal in this manner, characteristics (hfe and Early voltage, etc.) of transistors of the D/A converter circuits and the output circuits in the chips may be different. Therefore, it is difficult to make actual output currents of the chips precisely equal to each other. Further, since the reference current i is generated by the column driver IC 22 on the basis of the current Iout, which is one of the output drive currents of the column driver IC 21, a difference between the reference current i of the column driver IC 22 and the reference current IREF of the column driver IC 21 becomes large, so that the luminance variation in a boarder region on a display screen corresponding to an area between adjacent column driver ICs can not be removed sufficiently.
JP2003-28804SA entitled “Organic EL Drive Circuit and Organic EL Display Device” discloses a technique for solving such problem.
In the technique disclosed therein, a pair of resistors are provided within a column driver IC. A current from an output stage current source is supplied to one of the paired resistors and a current from an output current source of an upstream side column driver IC is supplied to the other resistor of the paired resistors. Voltages generated by the resistors according to these currents are compared with each other by an operational amplifier OP and the currents of the output stage current sources of the column driver IC are controlled to make them equal to each other by feeding back the currents in such a way that the voltages of the resistors become equal to each other.
On the other hand, due to the increase of the number of terminal pins, drive current variation between terminal pins becomes considerable. Therefore, more precisely defined drive currents are required. In view of this requirement, a problem occurs in the drive current control technique, in which paired resistors are utilized. That is, variation in resistance value of paired resistors influences on the drive current.
Particularly, when the drive current becomes smaller, an area of the paired resistors is increased necessarily, so that an area occupied by the column driver IC having such paired resistors is increased.
In the active matrix type current drive circuit, the drive current of an organic EL element is generated by charging a capacitor of a pixel circuit, which is, for example, several hundreds pF, with a current in a range from 0.1A to 10A. Therefore, requirements of S/N ratio and of preciseness of the drive current of the active matrix type organic EL drive circuit become more severe than those of the passive matrix type organic EL drive circuit.